TGFβ signaling regulates the response of the skeleton to phosphate

Inorganic phosphate (Pi) homeostasis is crucial to organismal health, yet the mechanisms underlying the regulation of it remain unclear. Critically, we lack a clear understanding of the Pi response circuitry in osteogenic cells that identifies altered serum Pi levels and transmits this information to changes in serum FGF23 levels, a key hormone regulating circulating Pi. We utilized genome-wide CRISPR screens in osteogenic Pi-responsive fluorescent reporter cell lines to identify regulators of the response to high phosphate, intersecting those results with loci associated with circulating FGF23 levels by genome-wide association studies (GWAS) and identified a potential role for TGF-β2. We found that each of the three ligands (TGF-β1, 2, 3) can enhance the response to Pi in osteogenic cell lines and ex vivo cultures of calvariae, while inhibitors of TGFβ receptor signaling dampen it. Co-treatment of Pi with TGFβ ligands led to an elevated, synergistic transcriptional induction of Slc20a1 , which encodes a key Pi importer, which corresponded with an increased intracellular uptake of phosphate. Furthermore, in mice, blocking TGFβ signaling disrupted the induction of FGF23 in mice on a high phosphate diet, resulting in disrupted downstream endocrine control of phosphate homeostasis. Together, these findings reveal a role for TGFβ signaling in the regulation of phosphate homeostasis in osteogenic cells through regulation of cellular phosphate uptake, which in turn contributes to the maintenance of organismal phosphate homeostasis.